Because of their excellent heat resistance, transparency, gas permeability and mold release characteristics, 4-methyl-1-pentene polymers have been employed for, for example, plates for electronic oven, release films for FPC, process paper for artificial leather, baking cartons, medical syringes, storage containers of medicines, agricultural chemicals, cosmetics, blood or the like, measuring equipment, and storage containers of perishable foods. The 4-methyl-1-pentene polymers, however, have poor heat sealing strength, and they have been desired to be further improved in the practical performance.
In order to impart heat sealing properties to the 4-methyl-1-pentene polymers, a method of laminating the 4-methyl-1-pentene polymers to other thermoplastic resins such as polyolefins has been studied. However, if the 4-methyl-1-pentene polymers are laminated to other thermoplastic resins, the gas permeability that is an excellent property inherent in the 4-methyl-1-pentene polymers is deteriorated. In addition, the 4-methyl-1-pentene polymers have a problem of low adhesion strength.
Under such circumstances as mentioned above, the present inventors have earnestly studied, and as a result, they have found that a laminate consisting of a layer of a 4-methyl-1-pentene polymer, an intermediate layer of an adhesive resin composition and a layer of a specific propylene polymer composition is excellent in not only heat sealing properties and gas permeability but also blocking resistance. Based on the finding, the present invention has been accomplished.
The present inventors have further found that a laminate consisting of a layer of a 4-methyl-1-pentene polymer, an intermediate layer of a specific adhesive resin composition and a layer of an olefin polymer is favorable for storage of vegetables and fruits.
It has been hitherto known that vegetables and fruits are packaged with packaging media to prevent rotting or deterioration or to keep freshness. For example, vegetables such as lettuce, sweet pepper, broccoli, asparagus, spinach and mushroom, or fruits such as peach are packaged with packaging media made of polypropylene, low-density polyethylene, polybutadiene and the like to keep freshness. However, it is particularly difficult to keep freshness of vegetables and fruits having high respiration rates, such as broccoli and asparagus, so that development of packaging media capable of keeping freshness for a long period of time has been desired.
It is generally said that use of packaging media having excellent gas permeability is desirable to keep freshness of vegetables and fruits having high respiration rates. In order to increase gas permeability of the packaging media, a method of thinning the packaging media can be thought, but if the packaging media are thinned, it becomes difficult to maintain strength appropriate for packaging media. Further, there is a limit to the thickness of the packaging media because production of thin packaging media is accompanied by technical difficulty. A method of incorporating a filler into a resin to increase gas permeability of the packaging media can also be thought, but there resides a problem of occurrence of pinholes in this method. In Japanese Patent Laid-Open Publications No. 168398/1993, No. 260892/1993, No. 22686/1994 and No. 264975/1995, a method of opening fine holes in the packaging media to increase gas permeability is described. This method, however, is undesirable from the hygienic viewpoint because there is a possibility of entering of bacteria through the holes.
On the other hand, it is said that vegetables and fruits having low respiration rates are desired to be packaged with packaging media having low gas permeability to keep freshness.
Thus, as materials of the packaging media for keeping freshness of vegetables and fruits, those having gas permeability corresponding to the respiration rate should be employed. In the prior art, however, it was difficult to control gas permeability of the packaging media over a wide range of high permeability to low permeability.
Under such circumstances as mentioned above, the present inventors have earnestly studied, and as a result, they have found that a laminate consisting of a layer of a 4-methyl-1-pentene polymer, an intermediate layer of a specific adhesive resin composition and a layer of an olefin polymer has excellent peel resistance and can be easily controlled in the gas permeability. Based on the finding, the present invention has been accomplished.